Paranoia is a disturbed thought process characterized by excessive anxiety or fear, often to the point of irrationality and delusion. Paranoid thinking typically includes persecutory beliefs concerning a perceived threat. In the original Greek, παράνοια (paranoia) simply means madness (para = outside; nous = mind) and, historically, this characterization was used to describe any delusional state.

Paranoia is distinct from phobia, which is more descriptive of an irrational and persistent fear, usually unfounded, of certain situations, objects, animals, activities, or social settings. By contrast, a person suffering paranoia or paranoid delusions tends more to blame or fear others for supposedly intentional actions that somehow affect the afflicted individual

In psychiatry, the term paranoia was used by Emil Kraepelin to describe a mental illness in which a delusional belief is the sole, or most prominent feature. In his original attempt at classifying different forms of mental illness, Kraeplin used the term pure paranoia to describe a condition where a delusion was present, but without any apparent deterioration in intellectual abilities and without any of the other features of dementia praecox, the condition later renamed schizophrenia. Notably, in his definition, the belief does not have to be persecutory to be classified as paranoid, so any number of delusional beliefs can be classified as paranoia. For example, a person who has the sole delusional belief that he is an important religious figure would be classified by Kraepelin as having 'pure paranoia'. More recently, the clinical use of the term has been used to describe delusions where the affected person believes they are being persecuted. Specifically, they have been defined as containing two central elements:

The individual thinks that harm is occurring, or is going to occur, to him or her. The individual thinks that the persecutor has the intention to cause harm.